Currently, third generation CDMA services for mobile communication systems such as mobile telephone systems are beginning to be provided, however, next generation mobile communication methods that will make communication even faster are being studied. In next generation mobile communication, together with increasing the transmission rate, reducing connection delays and transmission delays have become a large challenge.
In mobile communication, when a mobile station that is performing communication moves and the reception conditions becomes poor, the mobile station switches the base station with which it is communicating (handover). During the handover period, the mobile station is not able to receive data, so during that period, it is necessary for the network to save data intended for that mobile station in a buffer. In a conventional communication system, a host mobile control station (host station) saves data for a mobile station in an internal buffer according to a request from the base station with which communication is being performed (origin base station). After that, the host station continues to save data, and after the host station receives a buffer release request from the destination base station, the host station transfers the data for the mobile station that is saved in the buffer to the destination base station, and that destination base station sends that data to the mobile station.
FIG. 11 is a drawing explaining a conventional handover control sequence (refer to the non patent document 1).
A base station BS1 that is performing communication periodically requests a mobile station MS to measure the radio communication status and to send a report. After receiving the request to measure the radio communication status and send a report, the mobile station MS measures the reception levels from the surrounding base stations and sends a report to the base station BS1. The base station BS1 references the reported signal levels, and when it is determined that a handover (HO) is necessary, sets a destination base station.
Next, the origin base station BS1 sends a request to the destination base station BS2 to set the radio communication channel (HO request). When the origin base station BS1 receives a radio communication setting response in response to this HO request (HO confirmation), the origin base station BS1 requests the host station MCS to switch the communication base station from the origin base station BS1 to the destination base station BS2, and at the same time requests that the host station save data that is intended for the mobile station (buffer request). When there is space in the buffer and it is possible to switch from the origin base station BS1 to the destination base station BS2, the host station MCS saves the data for the mobile station in the buffer, and sends a switch OK signal to the origin base station BS1 (buffer confirmation OK). On the other hand, when there is no space in the buffer and it is not possible to switch from the origin base station BS1 to the destination base station BS2, the host station MCS does not send the buffer confirmation OK until switching becomes possible.
After there is a buffer confirmation response, the origin base station BS1 sends a request (HO instruction) to the mobile station MS to switch the communication base station from the origin base station BS1 to the destination base station BS2. The mobile station MS executes control according to this HO instruction to switch the communication base station from the origin base station BS1 to the destination base station BS2, and after switching is complete, sends a handover response (HO response) to the destination base station BS2. After receiving the HO response, the destination base station BS2 sends a buffer release request to the host station MCS. After receiving the buffer release request, the host station MCS transfers the data for the mobile station that is saved in the buffer to the destination base station BS2, and the destination base station BS2 sends that data to the mobile station. After that, the host station MCS sends data that is intended for the mobile station and received over a wired network such as an IP network to the destination base station BS2.
In the conventional handover control described above, the origin base station BS1 is able to send a handover instruction to the mobile station for the first time after a buffer confirmation response is returned from the host station MCS. In other words, the origin base station BS1 is not able to send a handover instruction to the mobile station until a switching OK (buffer confirmation OK) response is returned from the host station.
As was described above, when there is a buffer request from the origin base station BS1 to the host station MCS, the host station MCS does not respond with a buffer confirmation OK when it is not possible to switch from the origin base station BS1 to the destination base station BS2. Therefore, sending the handover instruction to the mobile station is delayed until switching becomes possible, so there is a problem in that the delay during handover becomes large.